MyPy and Vectors

NextTry.py

from __future__ import annotations  # type: ignore

from typing import Protocol, TypeVar, Generic


T = TypeVar("T")
U = TypeVar("U")
V = TypeVar("V")
W = TypeVar("W")

class Addable(Protocol[W]):
    def __add__(self, other: W) -> W: ...

class Vector(Protocol[T]):
    x: T
    y: T

    def __init__(self, x: T, y: T) -> None:
        ...

def free_add(a: Vector[Addable[V]], b: Vector[Addable[V]]) -> Vector[Addable[V]]:
    return a.__class__(a.x + b.x, a.y + b.y)


class RealVector(Generic[U]):
    def __init__(self, x: U, y: U) -> None:
        self.x = x
        self.y = y


free_add(RealVector(1,2), RealVector(3,4))
free_add(RealVector(1,2), RealVector("hi", "hello"))

SimpleTry.py

from typing import Protocol, TypeVar


T = TypeVar("T")

class Vector(Protocol[T]):
    x: T
    y: T

    def __init__(self, x: T, y: T) -> None: ...

def free_add(a: Vector[T], b: Vector[T]) -> Vector[T]:
    return a.__class__(a.x + b.x, a.y + b.y)

works.py

from __future__ import annotations  # type: ignore

from typing import Protocol, TypeVar, Generic


T = TypeVar("T", covariant=True)
U = TypeVar("U")
V = TypeVar("V")
W = TypeVar("W")

class Addable(Protocol[W]):
    def __add__(self, other: W) -> W: ...

class Vector(Protocol[T]):
    x: Addable
    y: Addable

    def __init__(self, x: T, y: T) -> None:
        ...

def free_add(a: Vector[V], b: Vector[V]) -> Vector[V]:
    return a.__class__(a.x + b.x, a.y + b.y)


class RealVector(Generic[U]):
    def __init__(self, x: U, y: U) -> None:
        self.x = x
        self.y = y


free_add(RealVector(1,2), RealVector(3,4))
free_add(RealVector(1,2), RealVector("hi", "hello"))

Just to be clear, these fail.